Process to recover a portion of post-recycling municipal solid waste

ABSTRACT

This specification describes a system and process for treating waste, for example municipal solid waste (MSW) or post-recycling municipal solid waste. The system includes a press and a pulper. The press is adapted to provide a wet fraction of the waste suitable for anaerobic digestion and rejects. The pulper is adapted to receive the rejects and produce pulp. The pulper may be, for example, a drum pulper. In the process, waste is separated into a press into an organic fraction and rejects. The organic fraction is treated by way of anaerobic digestion. The rejects are separated, optionally in a drum pulper, to produce a fraction containing pulp. The pulp can be re-used to make paper or other products. In some case, 70% or more or 80% or more of MSW can be diverted from landfill.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/445,347, Process to Recover a Portion ofPost-Recycling Municipal Solid Waste, filed on Jan. 12, 2017, which isincorporated herein by reference.

FIELD

This specification relates to solid waste processing.

BACKGROUND

Solid waste can be divided into various fractions distinguished, amongother things, by how easily they can be biodegraded. The organicfraction is the part of the waste that is most easily biodegraded andmay also be referred to as organic waste. The organic fraction isprimarily made up of food waste, but may also include leaf and yardwaste or other materials. The organic fraction is approximately 40% ofordinary municipal solid waste (MSW) after recyclables are removed.

Historically, organic waste was landfilled with other solid waste.However, the organic fraction of solid waste is the major cause ofgreenhouse gas emissions, leachate and odors in landfills. There is ageneral trend to divert organic waste for biological treatment, forexample by anaerobic digestion (AD) or composting. Most biologicaltreatment steps require some preprocessing of the waste such asdebagging and sorting to remove large items such as bottles and cans.Certain biological treatments, such as some composting methods andhigh-solids slurry and wet (low solids) anaerobic digestion, alsorequire that the waste be reduced in size and homogenized. The sizereduction is typically done in a device that comminutes the waste, suchas a hammer mill, shredder or pulper. In some cases, the comminutingdevice also provides a coarse separation of contaminants (i.e. materialthat is not readily biodegraded, such as plastic). Alternatively, aseparate separation device may be added.

Wet anaerobic digestion is typically performed in one or more mixedtanks. These systems are entirely contained and so allow for high levelsof odor control and biogas recovery. In many cases, the organic wastecan also be co-digested with wastewater treatment plant (WWTP) sludge bymodifying existing WWTP digesters rather than building new facilities.

US Publication 2013/0316428 describes an alternative process in which anorganic fraction containing biological cells is separated from solidwaste in a press. The organic fraction is extruded through a grid havingsmall-bore holes, under a pressure higher than the burst pressure of thecell membranes. The cells are disrupted and a gel or paste of a doughyconsistency is produced. The gel can be digested in an anaerobicdigester. The press may be as described in European Publication Nos.1207040 and 1568478 and Italian patent publication ITTO20111068. Ingeneral, these presses use a plunger to compress waste that has beenloaded into a cylinder. The sides of the cylinder are perforated withradial holes. US Publication 2013/0316428, European Publication Nos.1207040 and 1568478 and Italian patent publication ITTO20111068 areincorporated herein by reference.

INTRODUCTION

This specification describes a system and process for treating waste,for example mixed municipal solid waste (MSW) or post-recycling mixedmunicipal solid waste.

The inventors have observed that methods as described above do notdivert large amounts of mixed MSW from landfill in all cases.Comminuting devices treating MSW do not, generally speaking, producehigh quality products. Presses may divert, for example, 20-30% of themass of mixed MSW for efficient anaerobic digestion, but this stillleaves a large portion of the MSW for landfill.

A system described herein includes a press and a pulper. The press isadapted to provide a wet fraction of the waste suitable for anaerobicdigestion and rejects. The pulper is adapted to receive the rejects andproduce pulp. The pulper may be, for example, a drum pulper.

In a process described herein, waste is separated into a press into anorganic fraction and rejects. The organic fraction is treated by way ofanaerobic digestion. The rejects are separated, optionally in a drumpulper, to produce a fraction containing pulp.

Whereas treating mixed MSW with a pulper produces a low quality pulp,treating press rejects with a pulper produces pulp with fewercontaminants. Combining significant diversion of organic material viathe press to an anaerobic digester with further recovery of pulp divertsa significant amount of MSW from landfill. The pulp can be re-used tomake paper or other products. Anaerobic digestion of the wet fractionproduces biogas for fuel and optionally other products.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic process flow diagram of a waste processing system.

DETAILED DESCRIPTION

Recovery or large pieces of recyclable materials (i.e plastics, metals,cardboard and paper) from mixed municipal solid waste MSW is awell-established practice. There are several material recoveryfacilities (MRFs) that process mixed MSW as opposed to single streamwaste, which is separated for recycling at the source. These facilitiesare known in the industry as “dirty MRFs”. Several mechanical processesare used to recover recyclables from mixed waste. These processesinclude bag openers, shredders, screening, ballistic separators, windsifters, optical sorters, magnets, Eddy Current separators, and manualsorting. Removing and then recycling metals, OCC, paper and plasticsresults typically in 10 to 15% diversion of mixed MSW from landfill.

Mixed MSW also contains food waste and other organic materials.Typically the vast majority of the food waste contained in MSW passesthrough 6 to 10-inch (coarse) trommel or disc screens, along with othermaterials that do not have recycling value or that escaped the recyclingprocess upstream. These materials include mixed and soiled paper, brokenglass, textiles, grit and stones, wood, plastic film, and small sizeferrous and non-ferrous metals. Non-recyclable paper and other fiberscan account for as much as 20 to 30% of the coarse screeningunderfraction.

Wet organics can be recovered from the mixed MSW underfraction using anextrusion press as described for example in the patent publicationsdescribed in the background section above. The coarse screenunderfraction is suitable to feed to one or more commercially availablepresses such as an Organics Extrusion Press OREX 400, 500 or 1000 presssold by Anaergia. The extrusion press applies pressure on the waste in aconfined extrusion chamber that contains perforations. A portion of theorganic waste fluidizes under pressure and exits through the orifices toproduce a paste-like material. This paste-like material, which may becalled a wet fraction, is a suitable feedstock for anaerobic digestion(AD) or composting. The balance of the material fed to the press exitsas rejects. Organics recovery for digestion achieve by way of the pressprovides an additional 20 to 30% diversion in typical North Americanmixed MSW.

Many municipalities throughout North America and other parts of theworld require higher diversion of mixed MSW from landfill than whattraditional “dirty MRF” recycling can achieve even when coupled withorganics extraction for AD or composting. While the press rejects couldbe further processed into refuse derived fuel (RDF) for use as fuel forpower generation or cement kilns, thermal solutions such as this are notaccepted as landfill diversion in many communities, for example becauseof the carbon dioxide or other emissions associated with theseapplications.

The press rejects can contain up to 40% of paper and pulpable fiberswith no conventional recyclable value. However, after extracting foodwaste with the press, the press rejects can be fed to a pulper, forexample a drum or tub pulper as used in the pulp and paper industry, torecover the fibers in the paper and other cellulosic materials in therejects. In one example, a continuous drum pulper is used. The pressrejects may be fed directly to the pulper after being extracted from thepress.

The press rejects from treating mixed MSW are typically in the form ofclumps with 40 to 45% moisture content. The rejects are diluted near atthe inlet end of the drum pulper to about 20% solids with recirculatedwater heated to about 45 degrees C. The drum pulper produces a tumblingeffect at its inlet end that, after approximately 15 minutes, createspulp. The rotary drum of the pulper also contains a screening section atits outlet end. In this section the material is diluted to 4% solids andis washed as it is screened. The screen, for example with 15 mm holes,allows the pulp to exit. Along with the pulp, some grit, plastics andother non-pulpable materials smaller than 15 mm in size exit the screen.The screen overs exit at the end of the screen as washed pulper rejects.The pulp slurry that exits through the screen holes is preferablyfurther cleaned to produce a pulp suitable for use in one or more paperproduct such as backing board, cardboard, pressed cardboard dividers,etc. The further cleaning, alternatively called polishing, can consistof removing grit and metals in hydrocyclones with dilution to about 3%solids and further screening, for example to 1.5 mm or, depending on thepulp application, down to as low as 200 microns. After cleaning inscreens, the low (i.e. about 3%) solids pulp can be dewatered, forexample using a screw press. A screw press produces an about 30% solidspaper pulp cake that can be transported, for example in 1 tonsupersacks, for use as feedstock in pulp and paper mills. The screwpress filtrate can be used as dilution water in one or more of thepulping, screening and grit removal unit processes. Optionally, thefiltrate may treated with dissolved air flotation to remove suspendedsolids be fore reuse, which also helps avoid accumulation in theprocess.

Feeding un-pressed mixed MSW to drum pulpers has been attempted butwithout material success. The inventors believe that a high content offood waste exits the pulper along with the paper pulp and makes the pulpunsuitable as feedstock for pulp and paper mills. The use of anextrusion press before pulping improves the quality of the pulp andproduces a marketable pulp that is clean and suitable for various paperproducts such as cardboard. The combined used of an extrusion press thatextracts food waste organics and a pulper the treat the press rejectsafter organics extraction enables the recovery of valuable paper andother fiber in the form of a marketable pulp. Alternatively, the pulp ora portion of it can be sent to an anaerobic digester. While pulp is nothighly digestible on its own, the pulp can be pyrolyzed to produce moredigestible gasses or liquids or the pulp co-digested with food waste.

An example of a waste processing system and process flow through thesystem are shown in FIG. 1.

The combination of upstream materials recycling, food organicsextraction and paper pulping can, in at least some cases, results in 60%or more diversion of mixed MSW from landfill.

We claim:
 1. A solid waste treatment system comprising, a press; and, apulper, wherein the pulper receives rejects from the press.
 2. Thesystem of claim 1 wherein the pulper is a drum pulper or a tub pulper.3. The system of claim 1 wherein the press comprises a perforatedextrusion chamber and a corresponding plunger.
 4. A process for treatingsolid waste comprising the steps of, pressing the solid waste to producea wet fraction and rejects; and, extracting pulp from the rejects. 5.The process of claim 4 wherein the process of extracting pulp comprisesdiluting the rejects with heated water, tumbling the diluted rejects andscreening the diluted rejects.
 6. The process of claim 4 wherein thepulp is extracted using a drum pulper or a tub pulper.
 7. The process ofclaim 4 wherein the step of pressing comprises bursting cells in thewaste.
 8. The process of claim 4 comprising using the pulp to make apaper product.